A return-trap heating system closely resembles the vapor system except that the return trap provides a positive return of condensate to the boiler. This type can be used in all but the largest buildings if the equivalent direct radiation (EDR) capacity is not greater than the return-trap capacity.
Advantages of a return-trap steam-heating system include the following:
1. The pipe sizes may be smaller because of the higher steam pressures.
2. Return of condensate to the boiler is rapid and positive.
3. The system responds easily to thermostatic control.
4. Steam distribution may be balanced by the use of orifice valves.
There are some disadvantages to the return-trap heating system:
1. Steam circulation depends almost entirely on boiler pressure.
2. Sufficient head room above the boiler must be provided for boiler piping installation.
3. Physical limitations to the size and capacity of return traps exist, which limit the boiler capacity.
If a heating system is limited by the boiler water-level height, by boiler capacity, or because a return trap cannot be used, a condensate-return system may be installed. In this system, a condensation pump is installed to return the condensate to the boiler.
Condensate-return systems have the advantage of allowing the return lines to be located below the water level in the boiler and also allow high steam pressures. However, large drip traps and piping are required for variable-vacuum and vacuum-return-line systems.
Vacuum-return-line systems are similar to condensate-return-type installations except that a vacuum pump is installed to provide a low vacuum in the return line to return the condensate to the boiler.
Advantages of a vacuum-return-line steam-heating system include:
1. There is positive return of the condensate to the boiler.
2. Air is removed from the steam mechanically, resulting in a rapid circulation of the steam.
3. Smaller pipe sizes may be used because of the greater pressure differential between the supply and return lines.